Ultrasound imaging machines are an important tool for diagnosis and therapy in a wide range of medical applications. Historically, ultrasound machines were large, expensive units used only in radiology departments by highly trained specialists. To increase portability and enable ultrasound to be used at the point-of-care and by more users, various attempts have been made to reduce the size and cost of these systems. However, handheld or hand-carried devices are generally constrained in terms of the available power and space. To address these constraints, compromises are often made on the electronics. Such compromises tend to reduce temporal or spatial resolution, which, in turn, negatively affect image quality.
An ultrasound image is typically composed of multiple scanlines. A single scanline (or small localized group of scanlines) is acquired by transmitting ultrasound energy focused at a point in the region of interest, and then receiving reflected energy over time. The focused transmit energy is referred to as a transmit beam. A scanline can be parameterized using a start position and an angle. During the time after transmit, one or more receive beamformers coherently sum the energy received by different channels, with dynamically changing delays (or phase rotations), to produce peak sensitivity along the desired scanlines at ranges proportional to the elapsed time. Conventionally, beamforming delay values are pre-calculated and stored in memory. The resulting focused sensitivity pattern is referred to as a receive beam. The resolution of a scanline is a result of the directivity of the associated transmit and receive beam pair.
Ultrasound transducers typically comprise arrays of small piezoelectric elements. In some cases, a subset of the elements in an array are used to transmit or receive an ultrasound beam. Such subsets are respectively called transmit or receive “apertures”. Image quality can be improved by using a larger aperture.
A common method for reducing power consumption and circuit size is to reduce the number of front-end components such as analog-to-digital converters (ADCs) and amplifiers. However, fewer channels limits the maximum size of the receive aperture. This can undesirably result in reduced penetration and resolution of the ultrasound images.
A common method for improving image quality with fewer receive channels is to use a synthetic aperture. A synthetic aperture can be achieved by combining different sets of echoes from multiple similar transmit events. This yields a larger effective aperture which improves penetration and resolution. However, since a synthetic aperture requires multiple similar transmit events, it takes longer to acquire the data required to form each image. This can result in an undesirable decrease in frame rate.
Another method for improving image quality is pulse inversion harmonic imaging, also known as phase inversion imaging. This imaging method relies on the detection of harmonic echoes. Two pulses of similar amplitude but inverted phase are transmitted in close temporal succession. The resulting received signals are then combined, which causes linear scattering to cancel out, thus improving signal-to-noise ratio. Again, since multiple similar transmit events are required, this method also reduces the frame rate.
Frame rate can be improved by processing multiple receive beams for each transmit event. One way to process multiple beams in parallel is to provide multiple sets of beamformer hardware. Not only is this additional hardware expensive, but it is neither practical nor efficient in a portable system where physical space and power are limited. Furthermore, as the number of beamformers grows, the storage size required for pre-calculated beamforming coefficients grows as well, so that the number of beamformers is often limited by the amount of memory resources.
To address this memory limitation, beamforming coefficients can be calculated dynamically and either temporarily stored in a relatively small memory or provided in real-time. A number of such techniques are disclosed, for example, in U.S. Pat. Nos. 5,469,851, 5,653,236, and 8,211,018.
There remains a need for ultrasound system architectures and related methods that can provide a better combination of image quality, frame rate, and scalability. There is a particular need for such system architectures and methods which are suitable for use in handheld or hand-carried devices with embedded electronics.